API

MythBoardAPI

    static @NotNull MythBoardAPI getApi() {
        final MythBoardAPI api = Bukkit.getServicesManager().load(MythBoardAPI.class);
        if (api == null) {
            throw new IllegalStateException("MythBoardAPI is not registered. Please ensure the plugin is enabled!");
        }

        return api;
    }

    /**
     * Get MythBoard's implementation of the {@link CriteriaManager}
     * @return {@link CriteriaManager}
     */
    @Nullable CriteriaManager getCriteriaManager();

CriteriaManager

    /**
     * Register a criteria
     * @param criteria Your implementation of {@link CriteriaType}
     */
    void register(final @NotNull CriteriaType criteria);

    /**
     * Checks if a {@link CriteriaType} with this ID exists in the registry
     * @param id {@link String} ID of the {@link CriteriaType}
     * @return <code>true</code> if a {@link CriteriaType} exists in the registry, <code>false</code> if otherwise
     */
    boolean exists(final @NotNull String id);

    /**
     * Get a {@link CriteriaType} from the registry by its ID
     * @param id {@link String} ID for the {@link CriteriaType}
     * @return {@link CriteriaType} or <code>null</code> if it does not exist within the registry
     */
    @Nullable CriteriaType getCriteria(final @Nullable String id);

CriteriaType

    /**
     * Get the ID for this {@link CriteriaType}
     * @return {@link String}
     */
    @NotNull String getId();

    /**
     * Create an {@link CriteriaInstance} of this {@link CriteriaType}
     * @param options {@link Map}<{@link String}, {@link Object}>
     * @return {@link CriteriaInstance}
     */
    @NotNull CriteriaInstance create(final @NotNull Map<String, Object> options);

CriteriaInstance

    /**
     * Check if a {@link Player} meets this criteria
     * @param player {@link Player}
     * @return <code>true</code> if the {@link Player} meets the criteria, <code>false</code> otherwise
     */
    boolean meets(final @NotNull Player player);

Creating A Criteria

You can create your own custom criteria using the API!
More specifically, you'll be implementing your own criteria type.

public class ExampleCriteriaType implements CriteriaType {

    @Override
    public @NotNull String getId() {
        // Your criteria ID
        // Must be unique
        // Not case sensitive
        return "EXAMPLE"
    }

    @Override
    public @NotNull CriteriaInstance create(@NotNull Map<String, Object> options) {
        // The options map are the options in the scoreboard's config for this criteria!

        // It's important to practice safe casting and null checks
        String balanceString = (String) options.getOrDefault("balance", "0");

        double balance;
        try {
            balance = Double.parseDouble(balanceString);
        }
        catch (NumberFormatExecption exception) {
            // Handle exception
            balance = 0.0d;
        }

        // Return the conditions you want the player to meet
        // player is the player being checked against
        // You're actually returning a CriteriaInstance here
        return player -> {
            double playerBalance = ...;      // Get player's balance
            return playerBalance >= balance; // Return true if their balance is >= the required balance
        };
    }

}

Now it just comes down to registering your new criteria!
To achieve this, utilize the API's CriteriaManager.

public class YourMainClass extends JavaPlugin {

    @Override
    public void onEnable() {
        // Get the API
        MythBoardAPI api = MythBoardAPI.getApi();

        // null check
        if (api != null) {
            CriteriaManager manager = api.getCriteriaManager(); // Get the CriteriaManager
            manager.register(new ExampleCriteriaType());        // Register your CriteriaType
        }
    }

}